How good of a relationship do you have with your mother? My mother knows I no longer attend, she is mad zealous, and she is not shunning me nor will she. She is disappointed to say the least, but she is not shunning.
Of course i haven't dropped any serious bombs on her either. I suppose it depends. Do you want to give her answers that will amke the problem go away but aren't honest (as jgnat suggested), or do you want to tell her the truth, but just not all of it?
Those are questions only you can answer because you know your relationship with your mother and we don't.
No matter what, i would find out HOW she found this out. And of course there is always the right in the middle statement of....."Mom, I love you and appreciate your concern, but I don't really want to talk about it. I love God and my family. i don't want to affect anyones faith negatively. I don't really want to discuss it. So......how bout them Lakers. Dwight sure is a bust eh?"
That would be my humble suggestion. it is honest to the person who birthed you, but not shun inducing. :)